The emergence of video phones in the marketplace, coupled with the adoption and increasing implementation of Narrowband Integrated Services Digital Network (N-ISDN) standards, has brought to the attention of network and video telephony designers certain practical communications compatibility issues associated with the integration and co-existence of N-ISDN video telephony with "standard" telephony, also called Plain Old Telephone Service (POTS). As is well known, one N-ISDN standard is the Basic Rate Interface (BRI), which defines operating parameters for the transmission and reception of mixed medium digital information over a digital subscriber loop. For the basic rate interface, the loop is logically partitioned into two bearer (B) channels and one data (D) channel, commonly known as a 2B+D interface.
One of the major N-ISDN/POTS compatibility issues relates to the diverse types of calls that a caller using a multi-media terminal device can initiate to a called party whose subscriber loop characteristics and terminal device media support capabilities are unknown. For example, when a caller using an ISDN-compliant video phone wants to communicate with a called party whose access line arrangement and terminal device media support capabilities are unknown, the caller typically initiates an audio call using one of the BRI bearer channels as a voice grade communication path, since the caller is unable to ascertain whether the called party has a POTS line connected to an analog telephone set or an ISDN BRI subscriber loop connected to an ISDN-compliant video phone. In an ISDN environment, an audio call is initiated by an end-user device, such as a video phone requesting speech bearer service from the network. Similarly, a clear data call is established by an end user device, such as a video phone requesting unrestricted 64 Kbps bearer service from the network. The initiation of a voice grade call also stems from the caller's awareness that a clear data call to the called party will not be completed if the called party does not have an ISDN BRI subscriber loop.
If, in the course of their conversation, calling and called parties find out that they are both using the audio capabilities of ISDN-compliant video phones, it is likely that they might wish to switch to a multi-media (audio/video) call instead of the single medium (audio only) call. In that case, it would be desirable for the call to be transitioned gracefully from an audio only call to an audio and video call, without any loss of audio communication between the parties while the video call is being set up.
The ISDN standards developers anticipated this compatibility issue and accordingly, drafted CCITT Q.931 fallback negotiation standards specifications, also called "bearer capability selection standards", as a network-based solution to that problem. These standards include provisions for a call setup message to carry signaling information specifying, for example, a preferred medium and an alternate medium for a call. Thus, in an ISDN Q.931 fallback negotiation standard compliant networking environment, a video phone caller initiating a call to a party whose terminal capabilities are unknown would request an end-to-end clear data channel (for a video/audio call) as the preferred communication path, with an option for "fallback" to a voice grade channel as a communication path of last resort. The latter option is exercised when either the communication network connecting calling and called parties is unable to provide a clear end-to-end data channel connection, or the called party has a non-ISDN telephone set connected to a POTS line.
Since the Q.931 fallback negotiation standards involve appropriate circuit selection and other decisions by the originating, intermediate and terminating switches, these standards must be implemented in all of the switches within the communication path of a call in order to allow a communication network to reserve an application and medium independent transport mechanism for a clear data channel call. In that case, the terminal adapters or the terminal devices negotiate the speed, protocols, medium or application for the call. However, due to the high implementation costs associated with the Q.931 fallback negotiation standards, communications carriers have been reluctant to implement these standards in the switches deployed in their networks. With no clear sign on the horizon pointing to a speedy and widespread implementation of these standards, an alternate solution for gracefully transitioning a voice grade audio call to an ISDN audio and video call is needed.
Another attempt to provide a graceful transition from a one-medium call to a multimedia call involves the use of the premises-based H.221 ISDN standard, which includes specifications for dynamic reconfiguration of bandwidth allocation for different media within one ISDN bearer channel. More specifically, the H.221 ISDN standard offers capabilities to handle speed negotiation for each medium and handshake communications protocols between terminal devices before an end-to-end connection is transitioned from a one-medium call to a multimedia call. The H.221 standard offers an adequate solution for ISDN calls originating and terminating on video phones connected to the same ISDN switch, since the switch does not have to make a facilities selection decision to complete the call. Unfortunately, one of the limitations of the H.221 standard is that it represents a viable solution only for calls either purposely initiated over clear data channels or for calls that happen to use clear data connections by virtue of the fact that the originating and terminating video phones are connected to the same ISDN switch. Thus, a graceful transition from a one-medium call to a multi-media call is still an unresolved problem in a mixed ISDN and POTS communication network.